Vegetable-tanning materials

The apphcation of vegetable tanning materials has an additive effect on the leather. The more vegetable tannins appHed the more the leather becomes like vegetable-tanned leather. The color is changed, the fullness of feel increases, and the leather can be worked and embossed like vegetable leather. 

Vegetable-tanning materials in commercial quantities come from many different countries. Quebracho is a principal tanning material from South America. Wattie or Mimosa is suppHed from several African sources. India and other Asian countries supply a variety of materials including Myrabolans, Gall Nuts, and Tara Pods (3). 

The vegetable-tanning materials are commercially extracted using hot water. The extraction is normally done in countercurrent extractors that permit the final removal of the extracts with fresh water. The dilute extracts are then evaporated to the desired concentration in multiple effect evaporators. Some extracts may be further dried by spray drying or any other means that proves effective without overheating the extract. Extract preparation depends on the type of extract, the si2e of the operation, and the desired concentration of the final product. 

Howes, F. N. (1953), Vegetable Tanning Materials, Butterworth, London. 

Howes, E., 1953, Sumac. In Vegetable Tanning Materials, Butterworths Scientific Publications, London, pp 209-218. 

Howes, F.N., "Vegetable Tanning Materials" Butterworths, London,1939. 

Table 1. Vegetable tanning materials (selection, modified from Haslam 1989, Hausen 1981)...

Olivannan M S, Ramaswamy T 1971 Waterproofing of sole leather with basic aluminum sulphate dipping. Effect of pretanning with various vegetable tanning materials. Leather Sci (Madras) 18 186-195... 

The si2e of the vegetable tanning molecules and the coUoidal nature of the system result in the fixation in the hide of filling materials. The filling action is essentially an impregnation of the hide to form a dense firm leather. These properties are gready desired in sole and mechanical leathers. 

Vellum. There is a great deal to be said for this material which has proved durability and is certain to last much longer than any present-day vegetable-tanned leather. Difficulties can arise with full-vellum bindings... 

Materials used for leather dyeing are usually acid dyes, direct dyes, mordant dyes (Thorstensen, 1993) and reactive dyes (Shao and Zhao, 1984). Basic dyes are primarily used for dyeing vegetable tanned leather (Sandoz, 1949). 

The predominant tanning technique in the world is chrome tannage (about 85%). As a multi-purpose material chrome tanned leather can be transferred to leather with different properties (to different leather types) by further processing. However, a significant quantity of vegetable tanned leather is manufactured, too. Part-tanned chrome-free - often termed wet white - leather is also produced, in small but growing quantity. 

---